Explosive ordnance disposal (EOD) divers can be required to conduct multiple, long duration, random-depth-profile dives, which make the effective use of conventional diving decompression tables difficult. The complication of the decompression routine caused by erratic diving profiles exposes the diver to increased possibilities of decompression sickness (DCS). A Government laboratory, in an attempt to minimize DCS occurrence, has developed a diver-worn processor capable of monitoring both diving depth and time. This information along with a decompression algorithm can be used to compute decompression profiles on a real time basis. The underwater decompression computer has been reportedly compatible with the Intel 8080 microprocessor technology. It was not, however, designed to meet the EOD specifications of magnetic effects limits for non-magnetic equipment and its electronic components would be expected to draw electrical currents unacceptable for EOD applications.
This effort was directed toward the design and fabrication of an operational breadboard unit of an Underwater Decompression Computer (UDC) for use in the proximity of magnetic influence ordnance. The key efforts in this design were the utilization of state-of-the-art low-current drain integrated circuit technology (CMOS technology) and advances in depth transducer technology. These were used to develop a device based upon decompression data and algorithms, as presented in References 3-5. The design and development of the UDC and the fabrication of a breadboard unit, in addition to being oriented toward achieving an EOD-acceptable low magnetic influence, has included concern for the following: long battery lifetime; compatibility with Oxygen-Nitrogen and Oxygen-Helium gas diving mixtures; capability of determining depths to 300 feet of sea water (FSW); operability over a temperature range of 29 to 93 degrees Fahrenheit; depth determination accuracy of +2 feet, -1 foot of sea water; and capability of displaying the diver depth and the calculated safe ascent depth.
The decompression algorithm used in the advanced underwater decompression computer of the present invention has become largely empirical, and is essentially derived from that of computing decompression profiles by Braithwaite which evolved from Workman's approach. The diver's depth is sensed at two second intervals and inert gas uptake or elimination is calculated for nine tissue half-time compartments. During gas uptake the partial pressure of inert gas in the tissue is governed by an exponential whereas when gas elimination from tissue occurs, the gas washout is assumed to be linear.
The compartment half-times are associated with various perfusion-limited tissues and range from 5 to 240 minutes. At the end of the two second interval, the microprocessor output provides an updated inert partial gas pressure for each compartment. Decompression is regulated by comparing the updated partial pressure in the nine compartments to a table of "M-values" which are stored in a read only memory (ROM). An "M-value" is the maximum permissible inert gas tissue tension in any of the nine half-time compartments at a given depth. By comparing M-values and the calculated partial pressures which are updated at two second intervals, a safe ascent depth (SAD) can be determined.
To establish a SAD, the microprocessor compares the inert gas pressures in each of the nine compartments to corresponding M-values, and displays the shallowest depth at which all computed compartment gas tensions are less than or equal to their respective M-values. The diver's actual depth is also displayed simultaneously with the SAD. The action required by the diver during ascent decompression is to move up to the SAD and stay there until the SAD changes to the next more shallow depth. At that time, the diver again moves up to the SAD. This process repeates until the diver finally surfaces.
The UDC essentially maintains a history of the dive. Thus, as long as the diver does not turn off the UDC, he can move from lower to higher depths as frequently as necessary, as long as he does not rise above the displayed SAD. This provides the diver decompression capability for single and multiple dives and is one benefit of the UDC concept.
The original UDC included a program change of oxygen partial pressures whenever the diver was at a depth of 3 feet or less. This programmed capability which is also included in the advanced UDC permits the diver to breathe ambient air while surfaced instead of the 0.7 oxygen mixture of the MK-15 and MK-16 Underwater Breathing Apparatus.
The UDC algorithm has been implemented in FORTRAN simulations in both floating point and fixed integer formats. The floating point simulation stores all values in a 24 bit format, not including the exponent. This simulation produces a decompression schedule and values for minimum time to surface that agree completely with the published diving tables.
The fixed integer simulation which was necessary prior to microprocessor development has been operated with variable bit lengths. The minimum bit length to produce near agreement with the diving tables is 12 bits in the multiplicative arguments with 24 bits saved in the final tissue pressure result. The values produced by using this bit length were found to vary from the published tables for long periods of dive time. The reason is that the pressure increment added to each tissue pressure for a two second interval is very small, and at 24 bits the truncation error becomes significant over many iterations.
To eliminate this truncation problem, the UDC algorithm has been implemented in the UDC assembly code using 16 bits (two 8 bit bytes) for each multiplicative argument with 32 bits saved in the final tissue pressure result. This implementation has been tested and compared to the results of a 32 bit integer FORTRAN simulation, and the comparison is exact except in the least significant bit.
Certain types of fuzes in mines and other types of ordnance respond to magnetic field changes having certain temporal properties. Such fuzes typically null themselves to their ambient magnetic field so that field changes induced by select targets moving in proximity to the fuze will result in fuze actuation and ordnance detonation.
The detection and neutralization of this class of ordnance requires equipment (such as the UDC) and techniques which would not adversely perturb the ambient fields near such ordnance. The standard for materials and tools involves field perturbations dealing with quasi-static magnetic field perturbations attritubed to ferrous materials and steady state or slowly varying currents and eddy current generated magnetic fields. In order to minimize the static field perturbations resulting from ferrous materials, the materials used in EOD tools and equipment must have permeabilities close to unity. This condition is satisfied by most common alloys of aluminum, copper, and magnesium, as well as other materials such as leads and some stainless steels such as SS310. It is essential for the final UDC hardware that no ferrous impurities are contained in its materials and that fabrication procedures have not resulted in inadvertent magnetic property changes. To minimize steady state current effects, operating currents should not exceed one milliampere.
The eddy current generated magnetic fields are attributed to the circulating current induced in a conducting material by a time-varying magnetic field. Any moving conductor either in the form of a wire loop or simply a solid shape can, if not properly accounted for in the UDC design, induce unacceptable magnetic field perturbations. These effects need to be minimized by avoiding inductive loops in UDC electronic circuitry, by using eddy current minimizing laminations, by using materials having high electrical resistivity, and/or by using thin materials.
Pressure transducers typically rely on the measure of the deflection of one-side of a thin-walled structure. The deflection caused by the pressure changes on one side of such a structure can be correlated with the electrical signals using capacitive, inductive, piezoelectric, or peizoresistive techniques. Generally characteristics of these pressure sensing techniques are reported in Table I.
In general, recent advances in materials, fabrication, and packaging, combined with the rapid growth in signal processing capability have led to significant advances in pressure transducer performance at reduced cost. Many of these sensors utilize silicon
TABLE I ______________________________________ General Pressure Sensing Techniques CAPACITIVE Principle of operation Deflections of pressure diaphragm acting as one plate of a parallel plate capacitor cause capacitance changes. Pressure Range 0.01-200 psi Approximate Error 0.05% Advantages High accuracy and sensitivity; ruggedness; temperature insensitivity. Disadvantages High cost; unsuitability for high pressure. INDUCTIVE Principle of operation Deflections of pressure diaphragm or Bourdon tube cause inductance changes in inductance bridge or differential transformer. Pressure Range 0.04-10,000 psi Approximate Error 0.5% Advantages High outputs; wide pressure range. Disadvantages Instability with temperature; susceptibility to shock and vibration. PIEZOELECTRIC Principle of operation Pressure on a quartz or Rochelle-salt crystal produces an electrostatic voltage across it. Pressure Range 0.1-10,000 psi Approximate Error 1% Advantages No need for excitation; wide pressure, frequency response, and temperature ranges. Disadvantages Low output; temperature sensitivity. PIEZORESISTIVE Principle of operation Pressure induced strain in sensing element causes resistance change in guages. Pressure Range 0.5-10,000 psi Approximate Error 0.25-0.5% Advantages High sensitivity; low hysteresis and cost (semiconductor types); ruggedness; wide temperature range. Disadvantages Low output; temperature sensitivity. ______________________________________
circuitry with its accompanying problems related to its operation in various environments. Silicon circuitry has a tendency to be sensitive to adverse effects caused by temperature, moisture, magnetic fields, electromagnetic interference, and visible light. Thus, sensor packaging must allow the sensing element to be exposed to the environment while the on-board circuitry must be adequately protected. Many of the commercial sensors also draw electrical currents of tens of milliamperes which from a low magnetic influence and battery-drain perspective are unacceptable for the EOD UDC.
A review of the general characteristics of the various pressure sensing techniques in Table I readily shows that capacitive pressure transducers offer many of the advantages desired in the UDC. These include high accuracy, inherent ruggedness, and temperature insensitivity. Their principle disadvantage (i.e., high cost and unsuitability for high pressure) are currently relatively unimportant. The cited maximum pressure in their operating range of 200 psi corresponds to approximately 13-14 atmospheres or to a water depth measurement capability in excess of 400 feet of sea water (FSW). This depth exceeds the current UDC requirement to operate to depths of 300 FSW.
Silicon capacitive transducers have been developed by several organizations including Case Western Reserve University and Standford University. Commercially, Kavlico is producing several lines of capacitive transducers which incorporate microprocessor compatible signal conditioning electronics. Specifically, the Kavlico series P609 OEM pressure transducer utilizes an alumina capacitive sensing element, and has the specifications presented in Table II.
Although the current drain, operating voltage, and packaging for Kavlico devices were not directly amenable for use in the UDC, the ceramic alumina capacitive diaphragm was used as the basis for a low current drain (&lt;&lt;1 ma) transducer circuit design. Alumina is a well known ceramic which has been used frequently on military systems as an infrared dome and has well known, extensive media compatibility.
CMOS microprocessors which were reviewed for possible application in the advanced EOD underwater decompression computer included several 8 bit microprocessors and several 4 bit microprocessors. Overall evaluations of these microporocessors were made by considering the off-board/on-board RAM and ROM requirements, compatibility with LCD displays and available CMOS drivers, basic microprocessor architecture, development system hardware and software support requirements, total electrical current drain requirements, and capability for future hybridization/miniaturization. Based on these evaluations and the decompression algorithm requirements, it was decided to use the Motorola MC146805, a CMOS microprocessor that contains a powerful subset of the M6800 instruction set. This instruction set includes indexed addressing, true bit manipulation, versatile interrupt handling, and a useful set of branch instructions.
TABLE II ______________________________________ Kavlico Series P609 Pressure Transducer Specifications Pressure Ranges Standard: 0-5, 0-15, 0-30, 0-75, 0-150 psi absolute or gage; Custom ranges available. Proof Pressure 150% FS. Burst Pressure 300% FS or 300 psi, whichever is less. Pressure Media Any media compatible with silicon rubber (Buna N or Fluorosilicone available as an option), Valox 420, and 96% alumina. Cavity Volume Less than 0.15 cu. in. Volumetric Displacement Less than .001 cu. in. Pressure Connection 0-5 through 0-15 psi: 1/4" tube fitting or 1/4"NPT; 0-30 psi and higher: 1/4" NPT. Electrical Connection 3 solder lugs. Weight 4 ounces nomimal. Mounting Integrally molded flanges on T configuration; 1/4"NPT pressure connection on P configuration. Output 2.0 to 6.0 VDC standard @ 9 VDC input. Input Voltage 9.00 VDC operable from 8-12 VDC; Output ratiometric to input. Input Current Less than 10 ma. Output Impedance Less than 100 ohms. Load current 2 ma maximum. Output Ripple Less than 10 mV rms (at approximate- ly 1 KHz). Polarity Protection Protected against reverse voltage connection. Repeatability .+-.0.05% FS maximum. Hysteresis .+-.0.05% FS maximum. Linearity Better than .+-.1.6% FS. Operating Life More than 10 million cycles. Temperature Range -40 degrees C. to +85 degrees C. standard; (-55 degrees C. to +125 degrees C. optional). Temperature Stability Span: .+-.0.01% FS/deg C.; Zero: .+-.0.01% FS/deg C. Response Time 15 ms maximum (5 ms available). Body Injection molded Valox 420. Diaphragm and Substrate 96% alumina. Sensor-to-Housing Seal Silicone rubber (Buna N or Fluorosilicone available). Electronics Custom integrated circuit, hybridized, bonded to substrate and shielded. ______________________________________
This microprocessor is available in a version containing two kilobytes of on chip mask ROM or erasable programmable read only memory (EPROM).
Basically, the circuit design uses the changes in the decay time for the charged capacitor as part of a calibrated capacitor/resistor circuit to be correlated with sea water depth. In addition to requiring only extremely small currents for operation, this circuitry also is directly compatible with microprocessor logic, and does not require direct A-D conversion.
The pressure transducer circuitry may include a resistor the resistance of which will vary slightly with changes in temperature. Temperature changes will also affect the capacitive transducer, and the ideal RC circuit should match these two temperature variations in magnitude, but with opposite signs so that their temperature effects cancel.
During the developement of the UDC hardware and software, the microprocessor hardware was linked to a microprocessor development system through an in-circuit emulator. In order to test this hardware, a digital hardware simulation of the pressure transducer was built.
This transducer simulator receives a control pulse from the microprocessor hardware and returns an output pulse at a short time later exactly as the pressure transducer ordinarily would. The difference is that the time window between pulses is controlled by an 8 bit dip switch that can give time width from zero to a half second in discrete steps of 1/256th of a second. Using this simulator, the UDC microprocesor calibration software was tested, and some limited dive profiles were run to check the algorithm implementation, without actually using the pressure transducer.
The UDC decompression algorithm was coded both in FORTRAN and in MC146805 assembly code. The FORTRAN versions of the algorithm were merely simulations in order to establish that the algorithm that was implemented did indeed yield the same results as the published diving tables. Once this was established, then the FORTRAN simulation was converted to a fixed integer FORTRAN program that still yielded the same results, but that could be easily implemented in assembly code.
The FORTRAN algorithm is very compact: it requires only about fifty lines of uncommented code. However, this expands by an order of magnitude in the microprocessor assembly code. The entire software package resides in about 1650 bytes of EPROM. About half of the microprocessor code is used to implement the FORTRAN decompression algorithm and about half is used for I/O functions, interrupt routines, utility routines, initialization, etc.
The assembly code listings are well commented and follow the flow of the FORTRAN simulations directly. The results of the assembly code implementation agree with the FORTRAN results to within the least significant bit out of 32 bits used for one cycle through the code. Simulations have shown that agreement is required only down to the three least significant bits in order to match the published diving tables. This process of gradual translation of the decompression algorithm from a documented FORTRAN floating point simulation to the final assembly code implementation provides a higher level of confidence that the end result is operating as specified by the published diving tables.
The UDC software was developed on a Motorola EXORcisor II microprocessor development system using an in circuit emulator. With this configuration, the software was tested directly in the UDC hardware, and extensive software tools were available for program development and debug. The final version of the software was then burned into an EPROM and installed in the computer. Future modification of the software merely requires coding, testing using the UDC hardware with the in circuit emulator, and then burning a new EPROM with the updated software.
As described above, an advanced UDC intended for EOD applications and based on state-of-the-art low-current-drain integrated circuit (CMOS) electronic components has been bread-boarded. The breadboard hardware has been implemented using a Motorola MC146805 microprocessor and an eight-digit liquid crystal display (LCD). Actual diver sea water depth is determined by monitoring changes in the discharge time for a ceramic capacitive pressure sensor which is part of a calibrated RC circuit. This advanced UDC uses an algorithm developed by the U.S. Navy Experimental Diving Unit. The algorithm is based on a continuous updating of inert gas pressures in nine body tissues. The UDC displays a calculated diver safe ascent depth (SAD) in addition to the diver's actual depth.